B41oa oil and Gas Processing Section a flow Assurance Heriot-Watt University

Barium Sulphate Scale 
Of all of the various scales formed in oilfields, barium sulphate is a particularly 
difficult problem for two main reasons: 
1. Firstly, Barium sulphate has an extremely low solubility in aqueous 
solution (see Table 1). 
2. The second problem associated with the formation of barium sulphate 
relates to its extremely high thermodynamic stability and its hardness 
(mechanical strength) once formed. This makes chemical removal 
once formed (acidisation/scale dissolvers) often very difficult. 
Although the solubility of barium sulphate is low, it increases with increase in 
temperature, an increase in pressure and with an increase in salinity (of the 
particular brine system). 

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TOPIC 2: Oilfield Scale 
 
 
 
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NIVERSITY B41OA December 2018 v3 
The higher supersaturation of barium sulphate, when pressure and 
temperature are reduced, means that the severity of a barium sulphate scaling 
problem tends to increase in downstream equipment. 
Strontium Sulphate Scale 
Strontium sulphate generally follows similar trends to barium sulphate but it 
tends to be less of a problem due to its higher solubility. However, the 
strontium sulphate supersaturation becomes slightly reduced at reduced 
temperature. 
In general strontium sulphate co-precipitates with barium sulphate scales and, 
in a similar manner, calcium sulphates may also co-precipitate in mixed 
sulphate systems. 
Scale Aggregation 
An important feature of scale formation, both sulphate and carbonate scales, is 
that the scale itself is self-aggregating. For example, for barium sulphate and 
calcium carbonate scale formation, any drop in pressure leads to a small 
increase in the supersaturation (
P
S
). 
It follows that pressure reductions (for example at downhole choke vales) may 
cause scale formation. The build-up of scale provides a low energy surface for 
additional scale to form. 
Furthermore, the additional restriction caused by the build-up of scale at these 
points produces increased pressures drops at these points, which then further 
increases the scaling potential. 
In this manner, more severe scaling problems are often associated with 
downhole choke valves, safety valves etc. – resulting in serious well-
management and control problems.